Funds are requested to complete the redesign and testing of a second generation infrared heterodyne isotope ratiometer which measures the 13CO2 content of breath following administration of 13C-labeled substrates. These substrates are chosen for the metabolic or diagnostic significance of their oxidation rates. This instrument, called the Metabolic Carbon Analyzer, can be used in the analysis of discrete samples in an off-line mode, to provide diagnostic information on fat malabsorption, liver function, bacterial overgrowth of the small intestine, or the presence of Campylobacter Pylori infection in the gastric or duodenal mucosa. Moreover, the Metabolic Carbon Analyzer can be combined with conventional indirect calorimetry to provide live-time nonivasive measurements of substrate utilization in infants, children, pregnant or lactating women. This instrument will have a significant cost advantage over gas isotope ratio mass spectrometry and can be operated in a clinical research environment without special training. Completion of this developmental phase will enable the fabrication and deployment of 6 to 8 production prototypes. They will be placed in clinical centers for evaluation and field testing. The results will be used to confirm specifications for production of the commercial units.